Re: [PATCH v2] sched/rt: Fix double enqueue caused by rt_effective_prio

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Aug 03, 2021 at 12:45:01PM +0200, Juri Lelli wrote:
> From: Peter Zijlstra <peterz@infradead.org>
> 
> Double enqueues in rt runqueues (list) have been reported while running
> a simple test that spawns a number of threads doing a short sleep/run
> pattern while being concurrently setscheduled between rt and fair class.
> 
> WARNING: CPU: 3 PID: 2825 at kernel/sched/rt.c:1294 enqueue_task_rt+0x355/0x360
> CPU: 3 PID: 2825 Comm: setsched__13
> RIP: 0010:enqueue_task_rt+0x355/0x360
> Call Trace:
>  __sched_setscheduler+0x581/0x9d0
>  _sched_setscheduler+0x63/0xa0
>  do_sched_setscheduler+0xa0/0x150
>  __x64_sys_sched_setscheduler+0x1a/0x30
>  do_syscall_64+0x33/0x40
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> list_add double add: new=ffff9867cb629b40, prev=ffff9867cb629b40,
>                      next=ffff98679fc67ca0.
> kernel BUG at lib/list_debug.c:31!
> invalid opcode: 0000 [#1] PREEMPT_RT SMP PTI
> CPU: 3 PID: 2825 Comm: setsched__13
> RIP: 0010:__list_add_valid+0x41/0x50
> Call Trace:
>  enqueue_task_rt+0x291/0x360
>  __sched_setscheduler+0x581/0x9d0
>  _sched_setscheduler+0x63/0xa0
>  do_sched_setscheduler+0xa0/0x150
>  __x64_sys_sched_setscheduler+0x1a/0x30
>  do_syscall_64+0x33/0x40
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> __sched_setscheduler() uses rt_effective_prio() to handle proper queuing
> of priority boosted tasks that are setscheduled while being boosted.
> rt_effective_prio() is however called twice per each
> __sched_setscheduler() call: first directly by __sched_setscheduler()
> before dequeuing the task and then by __setscheduler() to actually do
> the priority change. If the priority of the pi_top_task is concurrently
> being changed however, it might happen that the two calls return
> different results. If, for example, the first call returned the same rt
> priority the task was running at and the second one a fair priority, the
> task won't be removed by the rt list (on_list still set) and then
> enqueued in the fair runqueue. When eventually setscheduled back to rt
> it will be seen as enqueued already and the WARNING/BUG be issued.
> 
> Fix this by calling rt_effective_prio() only once and then reusing the
> return value. While at it refactor code as well for clarity. Concurrent
> priority inheritance handling is still safe and will eventually converge
> to a new state by following the inheritance chain(s).
> 
> Fixes: 0782e63bc6fe ("sched: Handle priority boosted tasks proper in setscheduler()")
> Reported-by: Mark Simmons <msimmons@redhat.com>
> Signed-off-by: Peter Zijlstra <peterz@infradead.org>
> [squashed Peterz changes; added changelog]
> Signed-off-by: Juri Lelli <juri.lelli@redhat.com>

Thanks!